1. Field of the Invention
The present invention relates to semiconductor lasers and methods of fabricating such lasers. In particular, the present invention relates to digital versatile disks (hereafter referred to as "DVD") laser diode structure and fabrication methods.
2. Description of Prior Art
Semiconductor lasers are widely used in communication and data storage. For communication, an IR laser with a long wavelength is normally used. On the other hand, a laser with a relative short wavelength, such as an AlGaInP-based laser diode, is used for data storage.
AlGaAs-based laser diodes, which have a wavelength of 780 nm, are commonly used to record data on compact discs and CD-ROM discs. With the development of DVD, AlGaInP-based laser diodes are now in great demand since the wavelength of the laser light source used in the DVD specification is set to 630.about.650 nm.
The formula of AlGaInP alloy can be represented by (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P where x is a value between 0 and 1. The lattice of AlGaInP alloy is matched with that of GaAs. The AlGaInP alloy is a direct semiconductor when x is between 0 and 0.7. That is, the energy gap of Ga.sub.0.5 In.sub.0.5 P is 1.9 eV for x=0; the energy gap of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P is 2.3 eV for x=0.7. Therefore, the light-emitting AlGaInP-based devices emit red light. The (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P alloy, having a larger energy gap, can serve as a barrier layer in a quantum well, a waveguide or a cladding layer in the light-emitting device.
In the past, many structures and fabricating methods of AlGaInP-based laser diode have been proposed. Reference may be made to FIG. 1, which illustrates a cross section of a selectively buried ridge waveguide laser diode. Such a laser diode includes an n-type AlGaInP cladding layer 12, a GaInP active layer 14, a p-type AlGaInP cladding layer 16, an n-type GaAs blocking layer 18, a p-type GaInP layer 19, a p-type GaAs contact layer 20 and a p-type electrode 22 formed in this order on an n-type GaAs substrate 10. Then, an n-type electrode 21 is formed on the other side of the n-type GaAs substrate 10.
The structure mentioned above is fabricated by the MOCVD (Metalorganic Chemical Vapor Deposition) method, which grows an n-type AlGaInP cladding layer 12, a GaInP active layer 14, a p-type AlGaInP cladding layer 16, an n-type blocking layer 18 and a p-type GaInP layer 19 in order on an n-type GaAs substrate 10. A layer of Si.sub.3 N.sub.4 or SiO.sub.2 is then formed on the p-type GaInP layer 19. By photolithography and etching processes, a pattern of a strip having a width of 3.about.5 .mu.m is defined on the Si.sub.3 N.sub.4 or SiO.sub.2 layer. The strip of Si.sub.3 N.sub.4 or SiO.sub.2 layer is used to serve as a mask in the etching process. The deposited layers are then etched back to the p-type AlGaInP cladding layer 16 with the mask. By selectively applying MOCVD processes, a layer of n-type GaAs 18 is formed on the sidewall and the area except the strip area, i.e., the mask, to serve as a current blocking layer. A p-type GaAs contact layer 20 is then formed thereon after removing the strip mask.
Based on the above description, the fabrication process is very complex, because there are two regrowth steps in the fabrication process for a selectively buried ridge waveguide laser diode.
FIG. 2 shows another conventional semiconductor laser, a hetero-barrier blocking laser diode, which includes an n-type AlGaInP cladding layer 26, a GaInP active layer with quantum wells 28, a p-type AlGaInP cladding layer 30, a p-type GaInP cap layer 32, a p-type GaAs contact layer 34 and a p-type electrode 36 formed in this order on an n-type GaAs substrate 24, and an n-type electrode 35 formed on the other side of the n-type GaAs substrate 24.
The hetero-barrier blocking laser diode confines the current by the hetero-barrier blocking effect. That is, the current is confined by the difference between voltage drops of the p-type AlGaInP cladding layer 30/the p-type GaAs contact layer 34 and the p-type AlGaInP cladding layer 30/the p-type GaInP cap layer 32/the p-type GaAs contact layer 34.
Compared to the fabrication of a selectively buried ridge waveguide laser diode, a regrowth step can be reduced in the fabrication process of a hetero-barrier blocking laser diode. However, such structure has an inferior current confinement, and thus a low threshold current for the laser diode may not be obtained.
Referring to FIG. 3, the other conventional laser diode, a ridge waveguide laser diode, is provided. Such laser diode includes an n-type AlGaInP cladding layer 42, an active layer 44, a p-type AlGaInP cladding layer 46, a dielectric layer 48, a p-type GaInP layer 50, a p-type GaAs layer 52 and a p-type electrode 54 formed in this order on an n-type GaAs substrate 40, and an n-type electrode 53 formed on the other side of the n-type GaAs substrate 40. A ridge waveguide is formed on the p-type AlGaInP cladding layer 46, and both of the p-type GaInP layer 50 and the p-type GaAs layer 52 are formed on the ridge waveguide.
The above dielectric layer 48 can be nitride or oxide. Both of the n-type AlGaInP cladding layer 42 and the p-type AlGaInP cladding layer 46 can be represented by (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P, where x is 0.7. The active layer 44 has a quantum well structure, which consists of Ga.sub.y In.sub.1-y P and (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P formed in turn.
The ridge waveguide laser diode can be fabricated without the using of regrowth steps during the MOCVD process, but the etching of very small ridge width is difficult. Besides, the power output for single mode operation is limited.
As described above, the methods to fabricate AlGaInP laser diode of the prior art are too complicated and can be simplified by this invention.